Global Navigation Satellite Systems (GNSS) such as (NAVSTAR-)GPS (Global Positioning System) or the future European GNSS GALILEO use DSSS (Direct Sequence Spread Spectrum) modulation of the carrier frequencies reserved for the respective GNSS in order to transmit navigation data from the GNSS satellites of the space segment to GNSS receivers of the user segment. GPS and GALILEO commonly use the carrier frequencies L1 (1575.42 MHz) and L5 (1176.45 MHz). The DSSS modulation allows GNSS receivers to demodulate and decode the desired navigation signal on the carrier frequencies, which are commonly used by several GNSS satellites. The Russian GNSS GLONASS uses FDMA (Frequency Division Multiple Access) combined with CDMA so that each GLONASS satellite has its own two transmission frequencies for navigation signals. Regardless of the applied access technology, the navigation signals transmitted from GNSS satellites of GPS, GALILEO, and GLONASS are generally very weak at the receiver's location and, therefore vulnerable to jamming.
In mobile telecommunication systems, the use of FH combined with CDMA is applied in order to efficiently use the available resources for signal transmission. For example, the U.S. Pat. No. 6,215,810 B1 discloses a parallel hopping hybrid DS (Direct Sequence)/SFH (Slow Frequency Hopping) CDMA system, which combines the characteristics of a DS/CDMA, a FH/CDMA, a conventional CDMA and a multi-carrier CDMA system in order to be able to support more users of the system. FH has also been suggested as access technology for the transmission of navigation signals from GNSS satellites as being more resistant to multipath propagation.
FH has also some advantages with regard to jamming: if the sequence of hopping frequencies is unknown to unauthorized parties, it is significantly more difficult for these unauthorized parties to intercept the transmitted FH signals. Furthermore, the jamming of a FH signal is more demanding than the jamming of a signal with a constant carrier frequency: either, the hopping sequence must be known—which is usually not the case for unauthorized parties—in order to be able to tune the jamming signal accordingly, or the whole bandwidth within which the communication signal can be found must be jammed. This requires the emission of significantly more RF (Radio Frequency) power than for a constant carrier frequency, because the hopping frequencies can be chosen to cover a huge bandwidth, and increases the risk for the jammer to be detected. Thus, if navigation signals are transmitted as FH signals, these FH CDMA signals could be made less vulnerable to jamming. However, the processing of FH CDMA navigation signals requires a novel acquisition and tracking of these signals.
Noncoherent (slow and fast) frequency hopped spread spectrum signals, and hybrid and time hopping spread spectrum signals are introduced in chapter 1 of the book “Spread Spectrum Systems for GNSS and Wireless Communications”, Jack K. Holmes. Artech House 2007. ISBN 978-1-59693-083-4.